Mercury cathode alkali-chlorine cell containing a porous titanium or tantalum layered anode



July 29, 1969 can MERCURY CATHODE ALKALl-CHLORINE CELL CONTAINING A POROUS TITANIUM OR TANTALUM LAYERED ANODE Filed Nov. 21, 1966 INVENTUR. GOTTHARD CS IZI ATT'YS W W m Un wd W Pea- MERCURY CATHODE ALKALI-CHLORINE CELL CONTAINING A POROUS TITANIUM 0R TAN- TALUM LAYERED ANODE Gotthard Csizi, Bad Durkheim, Germany, assignor to Badische Anilin- & Soda-Fabrik Aktiengesellschaft, Ludwigshafen (Rhine), Germany Filed Nov. 21, 1966, Ser. No. 595,845

Claims priority, application Germany, Dec. 7, 1965,

Int. Cl. cizd 1/04 U.S. c1. 204-219 5 Claims This invention relates to novel anodes for horizontal alkali-chlorine cells with mercury cathodes.

It is known that electrodes consisting of a metal resistant to chlorine, e.g. titanium, tantalum or alloys of these metals, may be used as anodes in alkali-chlorine cells. Because of their high overvoltage these electrodes can however only be used if at least the side facing the counter-electrode has been activated. For this purpose the surface of the electrode is coated with metals of the platinum group, for example by electrolysis, vapor plating, or sintering. An electrode is thus obtained which consists of a carrier metal, e.g. titanium, to which a thin layer of a noble metal, e.g. platinum, has been applied. At this layer electrolysis takes place. The carrier metal may have any shape. It may be a perforated or unperforated sheet, a porous sintered article, or expanded metal. However, these electrodes have various disadvantages which have so far prevented them from being used in industry. The layer of noble metal is sensitive to amalgam, that is to say it is dissolved by sodium amalgam so that in some cases the active layer of noble metal is removed in a very short time. Moreover, cells provided with these electrodes have to be operated at as high a current density as possible because of their high cost. At high current densities, however, e.g. at more than 8K a./m. part of the effective anode surface is lost owing to the formation of a gas cushion, the result being that as the current density increases, the specific energy consumption per metric ton of product rises at such a rate that economic operation is no longer ensured.

It is an object of the present invention to provide a porous anode for use in horizontal alkali-chlorine cells with mercury cathodes and consisting of titanium, tantalum or alloys of these metals which does not have the said disadvantages.

This object can be achieved by using an anode consisting of layers of porous titanium, tantalum or alloys of these metals, the said layers being arranged one above the other in the direction of the cathode and the layer farthest from the cathode having been activated with a metal of the platinum group by a conventional method.

The design of the anode according to this invention prevents sodium amalgam from coming into contact with the sensitive layer of noble metal during electrolysis. The pore size of the layer facing the cathode must be such that, on the one hand, this is definitely prevented and, on the other, the brine can flow easily through the layer to reach the activated layer where the actual electrolysis takes place. It has been found that the pore size of this layer may be varied within wide limits according to the load on the cell. In general, pore sizes of from 100 to 800 microns, preferably 400 to 500 microns, are adequate to meet these requirements. The thickness of the layer may be 0.5 to 1 mm. Because of the high overvoltage of titanium no electrolysis takes place at this layer. The second metal layer is also porous and consists of the same metals. It has however been additionally activated with metals of the platinum group or alloys of these metals by a conventional method as mentioned above, e.g. by electrolysis, vapor plating or sintering. The pore size of this layer is so chosen that the capillary pressure of the brine is of the same order as the gas pressure. In general this requirement is satisfied by a pore size of from 15 to 200 microns, preferably 50 to microns. It is advantageous for the pore size of this layer to 'be less than the pore size of the first layer. The thickness of the layer should also be less than that of the first layer and may be between 0.3 to 0.5 mm. The second layer is connected.

to current supply lines.

According to a preferred embodiment the second activated layer has another porous layer applied to it which also consists of titanium, tantalum or alloys of these metals. The pores of this layer should be such that both the chlorine formed in the electrolysis and the brine which has been used up can pass through them. An average pore size of 200 to 800 microns meets this requirement. This layer may have a thickness of up to 10 mm. according to the electric conductivity required. Current is supplied through this layer to the intermediate fine-pored layer; it is therefore provided with a current supply means. This may consist for example of a copper rod tightly enclosed by a titanium tube which is closed at its lower end and which tightly fits into a recess in the third layer, the depth of this recess being less than the thickness of the third layer.

The electrode according to this invention is shown diagrammatically by way of example in the accompanying drawing.

The bottom layer 5 which faces the counter-electrode consists of porous titanium and is about 0.5 mm. in thickness. It has an average pore size of 300 microns. Electrolysis takes place at the layer 4 which has been applied to layer 5 by sintering. Layer 4 has smaller pores than layer 5; the average pore size is about 50 microns. It has been additionally activated by coating in a conventional manner with metals of the platinum group, e.g. an alloy of platinum and iridium. To this layer another coarse-pored layer 3 has been applied by sintering whose thickness is about 5 mm. and whose mean pore size is 500 microns. The joint between the two layers is electrically conductive. Layer 3 is provided with a recess 3 mm. in depth into which a copper tube 2 tightly enclosed by a titanium tube 1 has been forced. The titanium tube, whose diameter is about 600 mm., is closed at its lower end. This anode can be operated at a current density of up to 15,000 a./m. Accordingly, an amperage of up to 2,400 with an area of 0.16 m

One advantage of the electrode according to this invention is that it is not affected by the attack of sodium amalgam because the layer activated with elements of the platinum group is protected by the non-activated porous layer of titanium. Another advantage of the electrode is that it can be operated with less electricity or higher current density than prior art activated titanium anodes because its effective surface is not decreased by the formation of a gas cushion.

I claim:

1. A horizontal alkali-chlorine cell adapted to contain a flowing mercury cathode comprising an anode having at least two porous layers of titanium, tantalum or base,

alloys of these metals said layers being arranged one above the other in the direction of the cathode and the layer behind the layer facing the cathode being on its lower surface activated with a metal of the platinum group.

2. The cell as claimed in claim 1 wherein said layer facing the cathode has a pore size of 100 to 800 microns and said activated layer has a pore size of 15 to 200 microns.

3. The cell as claimed in claim 1 wherein said layer facing the cathode has a thickness of 0.5 to 1 mm. and said activated layer has a thickness of 0.3 to 0.5 mm.

4. The cell as claimed in claim 1 wherein another porous layer of titanium, tantalum or base alloys of these metals has been applied to said activated layer on the side away from the cathode, said another porous layer having a pore size at least as large as the pore size of said activated layer.

5. The cell as claimed in claim 1 wherein another porous layer of titanium, tantalum or base alloys of these metals hasbeenapplied to said activatedlayer on the side away from the cathode, said other layer having a pore size of 200 to 800 microns.

References Cited UNITED STATES PATENTS 2,788,460 4/1957 Santis et a1 117221XR 2,955,999 10/1960 Tirrell.

3,118,828 1/1964 Cotton etal.

3,222,265 12/1965 Beer 204-295 3,236,756 2/1966 -Beer.

3,385,780 5/1968 Feng 204 290 JOHN H. MACK, Primary Examiner D. R. JORDAN, Assistant Examiner Us. 01. X.R. 11722l; 204283, 290 

1. A HORIZONTAL ALKALI-CHLORINE CELL ADAPTED TO CONTAIN A FLOWING MERCURY CATHODE COMPRISING AN ANODE HAVING AT LEAST TWO POROUS LAYERS OF TITANIUM, TANTALUM OR BASE ALLOYS OF THESE METALS SAID LAYERS BEING ARRANGED ONE ABOVE THE OTHER IN THE DIRECTION OF THE CATHODE AND THE LAYER BEHIND THE LAYER FACING THE CATHODE BEING ON ITS LOWER SURFACE ACTIVATED WITH A METAL OF THE PLATINUM GROUP. 